Selective epitaxial growth techniques in the semiconductor fabrication industry are known. The use of selective epitaxial growth techniques is advantageous because it enables devices to be more closely spaced, yet still electrically independent of each other. This is accomplished by growing the epitaxial silicon from the single crystal silicon substrate through a seed hole which has been provided in a surrounding layer of dielectric material formed upon the substrate. The surrounding dielectric layer subsequently provides dielectric isolation between semiconductor devices formed on the epitaxial silicon.
Generally, the selectively grown epitaxial silicon has been formed in the following manner. A silicon substrate is first covered with an electrically insulating layer, typically silicon dioxide. This silicon dioxide layer subsequently forms the dielectric isolation layer. A seed hole is patterned in this silicon dioxide layer so as to expose the underlying silicon substrate within the seed hole. Using appropriate conditions of gas compositions, temperature and pressure, epitaxial silicon may be grown on the exposed substrate within the seed hole. No growth occurs on the surrounding silicon dioxide layer. This results in epitaxial silicon filling the seed hole to form a planar surface with the silicon dioxide layer. The silicon dioxide layer prevents signal leakage laterally between epitaxial silicon regions (or, perhaps, the devices formed in them). However, a shortcoming of this method is that leakage paths from the epitaxial silicon still exist through the underlying silicon substrate.
A technique has been proposed by the art to prevent these leakage paths through the underlying substrate using selective epitaxial growth techniques. This proposed method partially eliminates leakage paths through the substrate using epitaxial lateral overgrowth techniques. Epitaxial lateral overgrowth, or ELO, is simply selective epitaxial growth that is allowed to continue growing up and over the seed hole, and along the lateral surface of the surrounding dielectric layer. It is the lateral portions of epitaxial lateral overgrowth which are useful for forming devices because of the underlying oxide in those regions which provide dielectric isolation from the underlying silicon substrate. With this technique, there is still electrical continuity within the silicon regions through the seed hole. However, if desired, this may be eliminated by etching through the epitaxial silicon which overlies the seed hole.
The main shortcoming associated with epitaxial lateral overgrowth techniques is that it must be thinned by some means. The as-grown epitaxial lateral overgrowth is as tall as it is wide. However, most practical and useful devices require greater than five microns lateral distance and less than one micron vertical height for the silicon film to be feasible for forming devices. In order to form this desired structure, an etch-back or polishing step is required to thin the epitaxial silicon. This requirement for thinning of the laterally grown silicon is problematic because of the resulting poor uniformity across the substrate, low yields, non-standard equipment, and induced wafer stress.
In addition, another shortcoming associated with the conventional epitaxial lateral overgrowth techniques is that the seed hole must be located in the center of the epitaxial silicon region. There are many applications in circuit layout when it would be desirable to provide the seed hole in a region that is off-center.
It is therefore desirable to provide a method for forming laterally grown epitaxial silicon which does not require subsequent thinning, and which does not require the seed hole to be disposed in the center of the epitaxial silicon region. It is further desired that such a method be accomplished using standard semiconductor processing and manufacturing techniques.